Hello, first post here - looking for some help designing a circuit. I would call myself a beginner... I can build pretty simple things but have to look a lot up for more complicated designs.

I need help switching 250V on/off fast (sub microsecond). I know that photodiodes can be used as switches, but not quite sure how they actually work as switches. But I have a pretty basic idea of how I think I need to go about it. But not sure how to actually implement it.

What I planned on doing was using an external 5V trigger source to drive the leds.

When LED 1 is on, photodiode 1 sends 250V. When LED 2 is on, photodiode 2 sends 0V. Neither LED will be on at the same time.

The 250V will be coming from a 6kV supply that runs the rest of the set up.

Basically I just need a way to switch between 250V and 0V in 1 microsecond or less. And I was planning on driving that switch with a 5V TTL signal from a function generator.
I was just thinking that using leds and a photodiode was the easiest way to switch the 250V and 0V on/off fast with a function generator. If there's a bettwe way to do it I'm open to ideas.

okay... again sorry but this is something i am still trying to learn. i understand this all in principal, but actually soldering them to a board in a practical application is something i am still learning

but, i can use a fast optocoupler to drive the mosfet. and the mosfet will be fast (<1us) as well?

So hypothetically, i can use a 5v pulse to drive the optocoupler, and the output of the optocoupler will drive the mosfet which will turn my 250V on.

Now, the duration of the 250V, that can be adjusted by how long of a 5V pulse I give the optocoupler?

Say I want to turn on the 250 V for 1us, i send a 5v 1us pulse to the optocoupler.

If I want the 250V on for 1ms though, I can just increase the pulse length and the optocoupler will respond accordingly?

ok. so i am a huge noob with this and am trying to figure it out. i have an opto isolator and n channel mosfet. how exactly do i connect them?
I have a 5v pulse from a function generator that will turn the optoisolator on and off, so how do i connect the optoisolator to the mosfet to "drive" it?

I wasn't exactly sure what I totally might need, so I bought two of each of the following? I was getting 5V from the optocoupler, a previous post said I could use the optocoupler to drive a transistor.
But then I got thinking, why do I need the optocoupler at all, can't I just use the mosfet to switch the 250V on and off as quick as my trigger pulse width is?

No, it's not that. Most N-FETs need a gate voltage of about 10V to turn on fully and quickly. Logic level types will turn on fully with only 5V.
An opto-coupler is unlikely to provide enough current to charge/discharge the N-FET gate capacitance quickly enough directly, so a buffer transistor gate-driver stage would probably be needed.

No, it's not that. Most N-FETs need a gate voltage of about 10V to turn on fully and quickly. Logic level types will turn on fully with only 5V.
An opto-coupler is unlikely to provide enough current to charge/discharge the N-FET gate capacitance quickly enough directly, so a buffer transistor gate-driver stage would probably be needed.

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Okay, maybe I am looking at this the wrong way. When the signal is high, I need 250V to be on. When low, I need 0V to be on. And the switching time is <1us.

I haven't found an optocoupler that will handle that type of voltage and switch that fast.

Hello, first post here - looking for some help designing a circuit. I would call myself a beginner... I can build pretty simple things but have to look a lot up for more complicated designs.

I need help switching 250V on/off fast (sub microsecond). I know that photodiodes can be used as switches, but not quite sure how they actually work as switches. But I have a pretty basic idea of how I think I need to go about it. But not sure how to actually implement it.

What I planned on doing was using an external 5V trigger source to drive the leds.

When LED 1 is on, photodiode 1 sends 250V. When LED 2 is on, photodiode 2 sends 0V. Neither LED will be on at the same time.

The 250V will be coming from a 6kV supply that runs the rest of the set up.

You probably need to be looking in the general direction of a solid state relay - that takes care of the isolation, but it isn't easy to find a SSR sensitive enough for direct control by a photodiode - numerous manufacturers publish opto-app notes with examples of photdiodes connected to an op-amp.

Streetlights used to have a large cadmium-sulphide cell (LDR) rated at mains voltage for daylight switchoff, of course the LDR couldn't handle the lamp/ballast load - it controlled an electromechanical relay.

Cadmium sulphide LDRs seem to have pretty much vanished from catalogues since RoHS, but if your city council is replacing old inneficient streetlights - you might be in luck.

The only part I cannot figure out what to do with is the "20" and "21". I need the 250V/0V to be switched on a μs time scale using a small voltage pulse from a function generator.

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You might be best off using a valve, its harder to damage if you do some bit of the circuit wrong. Grounded grid is probably easiest, but you have to watch the cathode voltage connected to your solid stat part of the circuit (the photo diode wouldn't drive the cathode anyway).

Video amplifier transistors like the BF422/423 etc might go somewhere near the voltage you need to switch.

If you can get away with shunting the voltage as supplied by a current limiting resistor, you could consider the TO220 MOSFET as often found in set top boxes/VCRs/TVs/monitors etc - they are usually rated at least 600V and anything from 1A up.

The bigger the MOSFET; the bigger the gate capacitance, so if you want uS response you'll need a totem-pole gate driver that can both source and sink gate current. If you didn't need it to be so fast, a suitably biased photo diode could probably switch a typical MOSFET on and off according to light it receives.